Method of trimming ultra high strength hot stamping steel sheet using coining

ABSTRACT

The present invention may provide a method of trimming a ultra high strength hot stamping steel sheet using coining, which includes a forming step and a coining step which are simultaneously performed on a hot stamping steel sheet.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Korean Patent Application No. 10-2016-0032440, filed Mar. 18, 2016, the entire contents of which is incorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method of trimming a ultra high strength hot stamping steel sheet, and more particularly, to a method of trimming a ultra high strength hot stamping steel sheet using coining, which enables hard trimming to be carried out by a press by performing coining together with forming the ultra high strength hot stamping steel sheet.

Description of Related Art

Recently, environmental problems have become the focus globally, and all industries are seeking to find a method of reducing fuel in order to cope with the environmental problems. As solutions for achieving fuel reduction in automobile industries, there are presented a method of improving efficiency of an automobile engine and a method of reducing a weight of an automobile. The method of reducing a weight of a vehicle may be a good solution for improving fuel economy of an automobile. However, the reduction in weight of the automobile causes a problem in that strength and durability required for a vehicle are not satisfied. Therefore, a method of solving the problem becomes the greatest goal in the automobile industries.

Therefore, in the automobile industries, various environmentally-friendly vehicles are being developed for the purpose of reducing emission of carbon dioxide to 95 g/km which is about 27% of the current emission by 2021 based on the European regulations. In addition, automobile manufactures strive to develop downsizing technologies and technologies of improving fuel economy so that the corporate average fuel economy (CAFE, 54.5 mpg (23.2 km/l)) standards in force on 2025 are satisfied.

To this end, recent automobile industries make efforts to reduce a weight of a vehicle for the purpose of ensuring safety of passengers at the time of a vehicle collision, preventing environmental pollution, and improving fuel economy of a vehicle. As a solution for achieving the purpose, a hot stamping steel sheet has been applied to components that are mainly subjected to a vehicle collision, and the application of the hot stamping steel sheet is expected to be increased from now on.

The hot stamping steel sheet in the related art has been produced by blanking a steel sheet in the form of a coil, and then heating, pressing and cooling the blank material. The ultra high strength hot stamping steel sheet has been manufactured by completely transforming the blank material into an austenite structure by heating the blank material at a high temperature, and then creating a martensite structure by pressing the blank material using a die and rapidly cooling the blank material at the same time. However, because of high strength obtained after forming and cooling the hot stamping steel sheet, there was a problem in that it is impossible to cut the ultra high strength hot stamping steel sheet by using a press in the related art. Therefore, the ultra high strength hot stamping steel sheet produced in the related art has been cut by using a laser trimming technology. However, the laser trimming has caused low productivity, a use of high-priced laser trimming equipment, and a large amount of costs required for maintenance. For this reason, there was a problem in that the hot stamping steel sheet is not efficient in terms of costs and productivity compared to the steel sheet in the related art. Moreover, there were problems in that components may be corroded due to welding chips, spatters, and the like which are produced after a process of trimming the components because of the nature of laser, and the steel sheet is softened by heat from laser.

The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing a method of trimming a ultra high strength hot stamping steel sheet using coining, which reduces production costs of a ultra high strength hot stamping steel sheet, increases production efficiency, prevents corrosion, and prevents a steel sheet from being softened.

Technical problems to be solved by the present invention are not limited to the aforementioned technical problem, and other technical problems, which are not mentioned above, may be clearly understood from the following descriptions by those skilled in the art to which the present invention pertains.

To solve the aforementioned problems in the related art, the present invention provides a method of trimming a ultra high strength hot stamping steel sheet using coining, which includes a forming step and a coining step which are simultaneously performed on a hot stamping steel sheet.

In the present invention, a coining bushing, which is used for the coining step, may be mounted at an end portion of a flange.

In the present invention, the coining bushings may be mounted on an upper die and a lower die which are used for the forming step.

In the present invention, the coining bushing may be made of a material identical to a material applied to the upper die and the lower die which are used for the forming step.

In the present invention, the coining bushing may be attachable and detachable.

In the present invention, when a thickness of the hot stamping steel sheet is greater than 0 mm and equal to or smaller than 1.5 mm, the coining bushings may protrude to the outside of the dies at a height of 0.5 mm, respectively.

In the present invention, when a thickness of the hot stamping steel sheet is greater than 1.5 mm, the coining bushing may protrude to the outside of the die at a height which is 30% of the thickness the hot stamping steel sheet.

In the present invention, after the forming step and the coining step are simultaneously performed, hard trimming may be performed on the hot stamping steel sheet by using a press.

In the present invention, the method may further include before the forming step and the coining step are simultaneously performed on the hot stamping steel sheet, a material blanking step of blanking a steel sheet in the form of a coil, and a heating step of heating the blank material.

According to the method of trimming a ultra high strength hot stamping steel sheet using coining according to the present invention, it is possible to reduce production costs of a ultra high strength hot stamping steel sheet, improve production efficiency, prevent corrosion, and prevent a steel sheet from being softened.

The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of a laser trimming apparatus in the related art.

FIG. 2 is a photograph of scattered welding chips produced by laser trimming in the related art.

FIG. 3 is a photograph illustrating corrosion of a ultra high strength hot stamping steel sheet which occurs due to welding chips or spatters produced during a laser trimming process in the related art.

FIG. 4 is a step-by-step configuration diagram of a laser trimming method in the related art.

FIG. 5 is a step-by-step configuration diagram of a method of trimming a ultra high strength hot stamping steel sheet using coining according to an exemplary embodiment of the present invention.

FIG. 6 is a configuration diagram of a sheared surface of a typical steel sheet which is formed by hard trimming in the related art.

FIG. 7 is a configuration diagram of a sheared surface of a ultra high strength hot stamping steel sheet which is formed by hard trimming according to the exemplary embodiment of the present invention.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

Hereinafter, an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings. Terms or words used in the specification and the claims should not be interpreted as a general and dictionary meaning and should be interpreted as a meaning and a concept which conform to the technical spirit of the present invention based on a principle that an inventor can appropriately define a concept of a term in order to describe his/her own invention by the best method. Therefore, the exemplary embodiments disclosed in the present specification and the configurations illustrated in the drawings are just the best exemplary embodiments of the present invention and do not represent all the technical spirit of the present invention. Accordingly, it should be appreciated that various equivalents and modified examples capable of substituting the exemplary embodiments may be made at the time of filing the present application.

The present invention relates to a method of trimming a ultra high strength hot stamping steel sheet using coining. Recently, the ultra high strength hot stamping steel sheet is increasingly applied to a vehicle in order to ensure weight reduction and stability of the vehicle. In general, the ultra high strength hot stamping steel sheet is produced by blanking a steel sheet in the form of a coil, heating the blank material, and then pressing and cooling a hot stamping steel sheet 20. The steel sheet is formed by being heated at a high temperature, such that the steel sheet is easily formed, and the hot stamping steel sheet 20 is rapidly cooled in a die, such that the hot stamping steel sheet 20 has super high strength, and the residual stress caused by phase transformation is eliminated, such that dimension precision of the ultra high strength hot stamping steel sheet is improved.

However, the ultra high strength hot stamping steel sheet is impossible to be subjected to trimming (hard trimming) using a typical press due to high strength of a 120 K level or higher. FIG. 1 is a photograph of a laser trimming apparatus 17 in the related art. Because the ultra high strength hot stamping steel sheet is impossible to be subjected to hard trimming in the related art, a laser trimming process or the like was used as illustrated in FIG. 1. The laser trimming has problems in that productivity is low, and a large amount of process costs are required, that is, a large amount of costs for purchasing equipment and maintenance are required.

FIG. 2 is a photograph of scattered welding chips produced by laser trimming in the related art. FIG. 3 is a photograph illustrating corrosion of a ultra high strength hot stamping steel sheet which occurs due to welding chips or spatters produced during a laser trimming process in the related art. The laser trimming has problems in that as illustrated in FIG. 2, welding chips, spatters, and the like are produced after the process of trimming components because of the nature of laser, and as illustrated in FIG. 3, the components may be corroded due to the welding chips, spatters, and the like, and may be softened due to heat from laser.

To solve the aforementioned problems in the related art, the present invention provides a method of trimming a ultra high strength hot stamping steel sheet using coining, which includes a forming step and a coining step which are simultaneously performed on a hot stamping steel sheet 200. Coining bushings 107, which are used for the coining step, may be mounted at end portions of a flange, and the coining bushing 107 may be mounted on an upper die 101 and a lower die 103 which are used for the forming step. In addition, the coining bushing 107 may be made of a material identical to a material applied to the upper die 101 and the lower die 103 which are used for the forming step, and the coining bushings 107 may be attached to and detached from the upper die 101 and the lower die 103. Moreover, in a case in which a thickness of the hot stamping steel sheet 200 is greater than 0 mm and equal to or smaller than 1.5 mm, the coining bushings 107 may protrude to the outside of the dies at a height of 0.5 mm, respectively, and in a case in which a thickness of the hot stamping steel sheet 200 is greater than 1.5 mm, the coining bushings 107 may protrude to the outside of the dies at a height which is 30% of the thickness of the hot stamping steel sheet 200. Furthermore, after the forming step and the coining step are simultaneously performed on the hot stamping steel sheet 200, the hot stamping steel sheet 200 may be subjected to hard trimming by a press, and before the forming step and the coining step are simultaneously performed on the hot stamping steel sheet 200, a material blanking step of blanking a steel sheet in the form of a coil, and a heating step of heating the blank material may be further included.

FIG. 4 is a step-by-step configuration diagram of a laser trimming method in the related art. In the related art, after the blank is manufactured, the blank is conveyed to a hydraulic press while being heated from a room temperature up to 950° C. Thereafter, the heated blank is pressed by the hydraulic press, and an upper die 11 and a lower die 13 of the hydraulic press are cooled by a coolant 15 of a cooling apparatus installed in advance. Thereafter, the ultra high strength hot stamping steel sheet, which has been completely formed, is ejected, and then cut by a laser trimming apparatus 17. However, the ultra high strength hot stamping steel sheet, which is manufactured in the related art as described above, has a problem in that the ultra high strength hot stamping steel sheet cannot be cut by a typical press because of high strength thereof, and for this reason, the ultra high strength hot stamping steel sheet was cut by the laser trimming apparatus 17 as illustrated in FIG. 4. Therefore, to solve the aforementioned problems, the present invention provides a method of trimming a ultra high strength hot stamping steel sheet using coining in which a forming step and a coining step are simultaneously performed. FIG. 5 is a step-by-step configuration diagram of a method of trimming a ultra high strength hot stamping steel sheet using coining according to an exemplary embodiment of the present invention. In an exemplary embodiment of the present invention, a blank is manufactured by blanking a steel sheet in the form of a coil. Thereafter, the blank is heated up to 950° C. while being conveyed to a hydraulic press. Thereafter, as illustrated in the FIG. 4, the blank is pressed by the press dies having the coining bushings 107 mounted at end portions of flanges of the upper die 101 and the lower die 103 The coining is performed on the end portion of the flange simultaneously with the forming step. If the coining is performed after the forming step, there is a problem in that it is impossible to perform the coining because strength of the hot stamping steel sheet 200 becomes higher. The present invention solves the aforementioned problem by simultaneously performing the forming step and the coining step. In the forming step, the heated hot stamping steel sheet 200 is compressed by the coining bushings on the dies. Thereafter, the upper die 101 and the lower die 103 of the hydraulic press are cooled by a coolant 105 of a cooling apparatus installed in advance. Next, hard trimming is performed on the compressed and cooled hot stamping steel sheet 200 by using the press.

In the method of trimming the ultra high strength hot stamping steel sheet using coining according to an exemplary embodiment of the present invention, a material of the die used for the forming step may be SKD11, or other materials, which are typically used and subjected to vacuum heat treatment, may be applied as a material of the die. Moreover, the coining bushing 107, which is applied to the present invention, may be made of a material identical to a material of the die, and the coining bushing 107 may be attached to and detached from the upper die 101 and the lower die 103 according to an exemplary embodiment of the present invention. If a thickness of the hot stamping steel sheet 200 is 0 to 1.5 mm, the coining bushing 107 applied to the upper die 101 and the coining bushing 107 applied to the lower die 103 may protrude to the outside of the dies at a height of 0.5 mm, respectively. In addition, in a case in which a thickness of the hot stamping steel sheet 200 is greater than 1.5 mm, the coining bushing may protrude at a height which is 30% of the thickness of the hot stamping steel sheet 200.

FIG. 6 is a configuration diagram of a sheared surface of a typical steel sheet which is formed by hard trimming in the related art. It can be seen that in a case in which a steel sheet is sheared by a typical press in the related art, burrs are formed at a lower side of a sheared surface as illustrated in FIG. 6. FIG. 7 is a configuration diagram of a sheared surface of a ultra high strength hot stamping steel sheet which is formed by hard trimming according to the exemplary embodiment of the present invention. In an exemplary embodiment of the present invention, the forming step and the coining step are simultaneously performed, and as a result, indentations are formed at an end portion of the flange of the ultra high strength hot stamping steel sheet. It can be seen that in a case in which hard trimming is performed by using a press, burrs are not formed as illustrated in FIG. 7.

In an exemplary embodiment of the present invention, the forming step and the coining step, which use the press, are simultaneously performed on the hot stamping steel sheet 200, such that the hot stamping steel sheet 200 is cut by hard trimming using the press instead of laser trimming, and as a result, there is a great effect in producing the hot stamping steel sheet and components made by using the hot stamping steel sheet, costs are reduced by 50% compared to the related art because high-priced laser trimming equipment is substituted, and cycle time is increased by 500% in an exemplary embodiment of the present invention compared to laser trimming in the related art, such that productivity is improved by hard trimming which makes production efficiency and a production speed excellent. Moreover, it is possible to prevent components from being corroded by welding chips and spatters which are scattered when the laser trimming is performed, quality of the sheared surface is excellent compared to the related art, and it is possible to inhibit burrs from being formed.

For convenience in explanation and accurate definition in the appended claims, the terms “upper”, “lower”, “inner”, “outer”, “up”, “down”, “upper”, “lower”, “upwards”, “downwards”, “front”, “rear”, “back”, “inside”, “outside”, “inwardly”, “outwardly”, “interior”, “exterior”, “inner”, “outer”, “forwards”, and “backwards” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures.

The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents. 

What is claimed is:
 1. A method of trimming a ultra high strength hot stamping steel sheet using coining, the method comprising: forming and coining a hot stamping steel sheet.
 2. The method of claim 1, wherein a coining bushing, which is used for the coining, is mounted at an end portion of flanges of dies.
 3. The method of claim 2, wherein the dies include an upper die and a lower die and coining bushings are mounted on the flanges of the upper die and the lower die which are used for the forming.
 4. The method of claim 3, wherein the coining bushings are made of a material identical to a material applied to the upper die and the lower die which are used for the forming.
 5. The method of claim 3, wherein the coining bushings are attachable and detachable to the upper die and the lower die respectively.
 6. The method of claim 3, wherein when a thickness of the hot stamping steel sheet is greater than 0 mm and equal to or smaller than approximately 1.5 mm, the coining bushings protrude to the outside of the upper die and the lower die at a height of approximately 0.5 mm, respectively.
 7. The method of claim 3, wherein when a thickness of the hot stamping steel sheet is greater than approximately 1.5 mm, the coining bushings protrude to the outside of the the upper die and the lower die at a height which is approximately 30% of the thickness of the hot stamping steel sheet.
 8. The method of claim 1, wherein after the forming and the coining are simultaneously performed, hard trimming is performed on the hot stamping steel sheet by using a press.
 9. The method of claim 8, further comprising: before the forming and the coining are simultaneously performed on the hot stamping steel sheet, blanking a steel sheet in a form of a coil, and heating the blank steel sheet. 